Consider the following code:
struct S
{
S(int, double) {}
explicit S(const S&) {}
explicit S(S&&) {}
};
void i_take_an_S(S s) {}
S i_return_an_S() { return S{ 4, 2.0 }; }
int main()
{
i_take_an_S(i_return_an_S());
}
With the '-std=c++17' flag, both g++ and clang++ compile this code just fine. MSVC (with the /std:c++17 flag), however, reports
"error C2664: 'void i_take_an_S(S)': cannot convert argument 1 from 'S' to 'S'"
as a compilation error, with the additional note
"Constructor for struct 'S' is declared 'explicit'".
According to C++17's initialization rules (Explanation of point 3) S's copy constructor should not be considered for the initialization of the S parameter of i_take_an_S; S(int, double) should rather be selected as an exact match by direct-list-initialization.
Might this be a bug in MSVC?
Yes, MSVC seems to be wrong here.
Generally, since C++17, the initialization rules are so that S{ 4, 2.0 } will directly initialize the parameter S s of the function. (mandatory copy elision)
There is however an exception. An implementation is allowed to introduce a copy in a function parameter or a return value if the class type has only deleted or trivial copy/move constructors and destructor (and at least one of the former non-deleted).
That you declare the copy and move constructor explicit doesn't change that they are copy/move constructors. Because you are not using = default to define them, they are not trivial. Therefore the special permission does not apply and it is wrong of MSVC to try to perform a copy.
Furthermore this special kind of copy ignores accessibility and overload resolution and therefore explicit shouldn't be relevant even if it was performed, see [class.temporary]/3.
When exactly copy elision is performed affects the ABI however, so if this is a defect in MSVC's ABI, then it might not be easily fixed.